The demand for better and cheaper wireless telecommunication services and equipment continues to grow at a rapid pace. Part of this demand includes wireless message paging devices, which have become ubiquitous in society. Traditional one-way message paging devices (or "beepers") are giving way to newer two-way message paging devices. Additionally, the types of messages that may be send to a beeper have expanded from short telephone number messages to longer length alphanumeric messages and even to voice messages. In some systems, wireless messages may comprise an alphanumeric or voice message to which an electronic file, such as a text document, may be attached.
Much of this growth resulted from the Federal Communication Commission's ("FCC") approval of certain frequency bands for the next generation of Personal Communication Service ("PCS") devices that provide voice telephone service as well as advanced voice and/or data message paging services. A relatively small portion of the available spectrum was set aside for narrowband PCS, which is more suited to advanced message paging services, to encourage efficient use of the available spectrum.
Despite the wider use of higher precision electronics, the implementation of time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA) technologies, and the advent of narrowband PCS services, traditional problems associated with wireless messaging still persist. Paging messages are frequently not delivered to a subscriber. Part of the reason for this is that RF signal obstructions, RF noise, and multipath delay fading remain significant hindrances to wireless messaging systems.
The structure of traditional message paging systems have been somewhat different than cellular telephone systems. In a message paging system, all of the base station transmitters throughout a wide coverage area are synchronized and simultaneously broadcast (i.e., simulcast) a burst of many paging messages in a forward-channel to the paging devices of the system subscribers. This simulcast increases the likelihood that the paging message will reach the pager even through obstacles, such as buildings. Generally, paging systems do not assign each subscriber to a cell and transmit to the subscriber only in that cell, as in the case of a cellular telephone system.
A paging device that is situated relatively close to a base station transmitter generally receives a strong signal from the nearby transmitter that overpowers time-delayed, but much weaker, signals from more remote transmitters. However, as a subscriber moves from the coverage area of the nearby base station transmitter toward the coverage area of a more remote transmitter, the paging device necessarily moves through an intermediate region in which the signals from both base station transmitters are of comparable magnitude. In this region it is important to minimize the amount of delay spread between the two signals.
The traditional method of dealing with delay spread is to synchronize as accurately as possible the launches of paging messages from all transmitters. This may be accomplished in a variety of ways. For instance, in a paging system using a satellite downlink, a paging message queue can be transmitted by the satellite to a group of base station transmitters covering a certain coverage area. A memory in each base station transmitter stores time proportional to the exact distance between the base station transmitter and the satellite. Each base station transmitter can then use this (time difference) to calculate the "time-of-flight" delay from the satellite to the base station transmitter. Accordingly, base station transmitters that are closer to the satellite can delay the launch of the paging message queue to compensate for the longer time-of-flight delays of more remote base station transmitters. In this manner, the burst of paging messages are launched nearly simultaneously from the base station transmitters, and delay spread is minimized in regions approximately midway between base station transmitters.
In wired paging message systems, a wired backbone is used to deliver paging message queues from a central paging server to the base station transmitters. In these systems, a timing signal may be used to trigger the launch of the paging messages. Similarly to satellite-based systems, a memory in each base station transmitter stores the delay times, which correspond to distances, between the base station transmitter and the central paging server. Each base station transmitter can then use the wire length to calculate the signal delays from the central paging server to the base station transmitter. Accordingly, base station transmitters having short signal delays can delay the launch of the paging message queue to compensate for longer signal delays experienced by more remote base station transmitters. Alternatively, the base station transmitters may use an external clock signal to trigger the launch of the paging message queues, such as a clock signal from the global positioning satellite (GPS) system.
The above-described methods of synchronizing the launch of paging messages from a plurality of base station transmitters in a paging coverage area in order to minimize the effects of delay spread are still not sufficient to eliminate all problems related to delay spread. Frequently, terrain features and man-made objects may obstruct some signals and reflect others. These factors may have the combined effect of attenuating the comparatively strong signals from nearby base station transmitters relative to the comparatively weak (and time-delayed) signals received from more remote base station transmitters. This is particularly true if remote transmitters are located on high ground, such as on surrounding mountain tops. Thus, at any given point in a paging system, a paging device may still receive time-delayed versions of the same signal. These signals may destructively interfere with each other, even though each signal may separately be a good usable signal.
To further reduce the delay spread problem, some paging providers increase the number of base station transmitters in a paging coverage area and use down-tilt antennas to concentrate more of the transmitted power in the regions closer to the antenna. This effectively increases the relative strengths of signals received by a paging device from nearby base station transmitters compared to the relative strengths of signals received from more remote base station transmitters. However, this approach greatly increases the amount of infrastructure required to cover a region. Correspondingly, the equipment cost of the messaging paging system rises, as does the monthly fee paid by each subscriber.
Therefore, there exists a need in the art for an improved wireless communication system that minimizes the problems associated with the delay spread of simulcast signals. In particular, there is a need for an improved wireless communication system that minimizes delay spread problems without significantly increasing the equipment cost necessary to operate the system.